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Abstract:

An energy-saving magnetic bearing device with no bias current for making
the relation between the excitation current and the magnetic force of the
electromagnet linear is provided. In a magnetic bearing device for
supporting a rotor 1 serving as the magnetic piece in a levitating state
allowing free rotation at a specified position by the magnetic force of a
pair of electromagnets 2, 3, the electromagnets 2, 3 are constituted to
interpose the rotor 1 and face each other. A driver 204 is a PWM (pulse
width modulation) type driver for controlling the excitation current in
the electromagnets 2, 3 by modulating the pulse width of a voltage driven
at a specified carrier frequency fc, and includes a resonator means for
electrically resonating at a frequency equal to the carrier frequency fc.
When an excitation current flows in either one of the pair of opposing
electromagnets 2, 3, then the other magnet is regulated so that the DC
component in the electromagnet excitation current is zero, and a voltage
is applied via the resonator means to the electromagnet whose DC
component in the excitation current is discharged to zero.

Claims:

1-34. (canceled)

35. A magnetic bearing device for rotatably supporting a magnetic piece in
a levitating state at a specified position comprising:an electromagnet
for supporting the magnetic piece in a levitating state by magnetic
force;displacement detection means for detecting a displacement of the
magnetic piece based on impedance change in the electromagnet; anda
driver for applying an excitation current to the electromagnet so as to
support the magnetic piece in a levitating state at a specified position
based on a detection signal from the displacement detection means;wherein
the magnetic piece is interposed between a plurality of electromagnets
installed opposite each other,the driver is a pulse width modulation type
driver to control an excitation current of the electromagnet by
modulating a pulse width of a pulse voltage driven at a specified carrier
frequency, and includes an alternating current transfer means for setting
a direct current component of the excitation current to zero, and when
excitation current flows to one of the electromagnets installed facing
each other, then the alternating current transfer means sets a direct
current component of the excitation current of the other electromagnet to
zero.

36. A magnetic bearing device for rotatably supporting a magnetic piece in
a levitating state at a specified position comprising:an electromagnet
for supporting the magnetic piece in a levitating state by magnetic
force;displacement detection means for detecting a displacement of the
magnetic piece based on impedance change in the
electromagnet;compensation means for compensating so as to stably support
the magnetic piece in a levitating state based on a detection signal from
the displacement detection means; anda driver for applying an excitation
current to the electromagnet based on an output signal from the
compensation means;wherein the magnetic piece is interposed between a
plurality of electromagnets installed opposite each other,the driver is a
pulse width modulation type driver to control an excitation current of
the electromagnet by modulating a pulse width of a pulse voltage driven
at a specified carrier frequency, and includes an alternating current
transfer means for setting a direct current component of the excitation
current to zero, and when excitation current flows to one of the
electromagnets installed facing each other, then the alternating current
transfer means sets a direct current component of the excitation current
of the other electromagnet to zero.

37. The magnetic bearing device according to claim 36, wherein the
compensation means is means to compensate a control loop phase.

38. A magnetic bearing method for rotatably supporting a magnetic piece in
a levitating state at a specified position with a plurality of
electromagnets installed opposite each other interposing the magnetic
piece therebetween comprising the steps of:supporting the magnetic piece
in a levitating state by magnetic force of the electromagnet;detecting a
displacement of the magnetic piece based on impedance change in the
electromagnet; anddriving for applying an excitation current to the
electromagnet so as to support the magnetic piece in a levitating state
at a specified position based on a detection signal obtained by the
detecting step;wherein the driving step has a pulse width modulating step
to control an excitation current of the electromagnet by modulating a
pulse width of a pulse voltage driven at a specified carrier frequency,
and includes an alternating current transferring step for setting a
direct current component of the excitation current to zero, and when
excitation current flows to one of the electromagnets installed facing
each other, the alternating current transferring step sets a direct
current component of the excitation current of the other electromagnet to
zero.

39. A magnetic bearing method for rotatably supporting a magnetic piece in
a levitating state at a specified position with a plurality of
electromagnets installed opposite each other interposing the magnetic
piece therebetween comprising the steps of:supporting the magnetic piece
in a levitating state by magnetic force of the electromagnet;detecting a
displacement of the magnetic piece based on impedance change in the
electromagnet; andcompensating so as to stably support the magnetic piece
in a levitating state based on a detection signal obtained by the
detecting step; anddriving for applying an excitation current to the
electromagnet so as to support the magnetic piece in a levitating state
at a specified position based on a result from the compensating
step;wherein the driving step has a pulse width modulating step to
control an excitation current of the electromagnet by modulating a pulse
width of a pulse voltage driven at a specified carrier frequency, and
includes an alternating current transferring step for setting a direct
current component of the excitation current to zero, and when excitation
current flows to one of the electromagnets installed facing each other,
the alternating current transferring step sets a direct current component
of the excitation current of the other electromagnet to zero.

40. The magnetic bearing method according to claim 39, wherein the
compensating step is a step of compensating a control loop phase.

41. The magnetic bearing device according to claim 35, wherein the
alternating current transfer means is resonator means with a serially
connected coil and capacitor for electrically resonating at the same
frequency as the carrier frequency.

42. The magnetic bearing device according to claim 36, wherein the
alternating current transfer means is resonator means with a serially
connected coil and capacitor for electrically resonating at the same
frequency as the carrier frequency.

43. The magnetic bearing device according to claim 35, comprising:means
for detecting respective excitation currents flowing in the
electromagnets installed opposite each other sandwiching the magnetic
piece and subtracting a deference of signals of the detected currents
after the deference signal passing through filter means of specific
frequency characteristics from the output of the displacement detecting
means, wherein the specific frequency characteristics are equivalent to
the frequency characteristics of the displacement detection means.

44. The magnetic bearing device according to claim 36, comprising:means
for detecting respective excitation currents flowing in the
electromagnets installed opposite each other sandwiching the magnetic
piece and subtracting a deference of signals of the detected currents
after the deference signal passing through filter means of specific
frequency characteristics from the output of the displacement detecting
means, wherein the specific frequency characteristics are equivalent to
the frequency characteristics of the displacement detection means.

45. The magnetic bearing device according to claim 35, wherein a signal
contained in a signal detected by the displacement detection means other
than the displacement information of the magnetic piece is removed by AM
modulating the carrier frequency signal with an output signal of a filter
means of characteristics equivalent to the transfer characteristics from
getting current flow to the driver to current flowing in the
electromagnets and by subtracting a signal obtained by the AM modulation
from a signal obtained from comparing a value of a ripple current
occurring after applying a pulse voltage to an electromagnets in which
excitation current is flowing, among an electromagnet pair installed
facing each other, with a value of a ripple current occurring after
applying a voltage via the alternating current transfer means to the
other magnet whose direct current component within the excitation current
is set to zero.

46. The magnetic bearing device according to claim 36, wherein a signal
contained in a signal detected by the displacement detection means other
than the displacement information of the magnetic piece is removed by AM
modulating the carrier frequency signal with an output signal of a filter
means of characteristics equivalent to the transfer characteristics from
getting current flow to the driver to current flowing in the
electromagnets and by subtracting a signal obtained by the AM modulation
from a signal obtained from comparing a value of a ripple current
occurring after applying a pulse voltage to an electromagnets in which
excitation current is flowing, among an electromagnet pair installed
facing each other, with a value of a ripple current occurring after
applying a voltage via the alternating current transfer means to the
other magnet whose direct current component within the excitation current
is set to zero.

47. The magnetic bearing device according to claim 35, further comprising
a linearization means for changing the non-linear relation between the
magnetic force of the electromagnet exerted on the magnetic piece and the
excitation current into a linear relation.

48. The magnetic bearing device according to claim 36, further comprising
a linearization means for changing the non-linear relation between the
magnetic force of the electromagnet exerted on the magnetic piece and the
excitation current into a linear relation.

49. The magnetic bearing device according to claim 47, wherein the
compensation means, or the linearization means, or the compensation means
and the linearization means acquire an output by digital processing.

50. The magnetic bearing device according to claim 48, wherein the
compensation means, or the linearization means, or the compensation means
and the linearization means acquire an output by digital processing.

51. A magnetic bearing device for rotatably supporting a magnetic piece in
a levitating state at a specified position comprising:an electromagnet
for supporting the magnetic piece in a levitating state by magnetic
force;a displacement detection means for detecting a displacement of the
magnetic piece based on impedance change in the electromagnet; anda
driver for applying an excitation current to the electromagnet so as to
support the magnetic piece in a levitating state at a specified position
based on a detection signal from the displacement detection means;wherein
the driver is a pulse width modulation type driver to control an
excitation current of the electromagnet by modulating a pulse width of a
pulse voltage driven at a specified carrier frequency, andthe
displacement detection means has a ripple detection means for detecting
an amplitude of a ripple current generated by the driver applying a
voltage to the electromagnet, and a signal contained in a signal detected
by the displacement detection means other than the displacement
information of the magnetic piece is removed by subtracting a signal
obtained from an output signal of the displacement detection means
passing through a filter means of characteristics equivalent to the
transfer characteristics from getting current flow to the driver to
current flowing in the electromagnets and through an amplifying means for
amplifying with a specified gain, from a signal obtained from the ripple
detection means.

52. A magnetic bearing device for rotatably supporting a magnetic piece in
a levitating state at a specified position comprising:an electromagnet
for supporting the magnetic piece in a levitating state by magnetic
force;a displacement detection means for detecting a displacement of the
magnetic piece based on impedance change in the electromagnet;a
compensation means for compensating so as to stably support the magnetic
piece in a levitating state based on a detection output signal from the
displacement detection means; anda driver for applying an excitation
current to the electromagnet based on an output signal from the
compensation means;wherein the driver is a pulse width modulation type
driver to control an excitation current of the electromagnet by
modulating a pulse width of a pulse voltage driven at a specified carrier
frequency, andthe displacement detection means has a ripple detection
means for detecting an amplitude of a ripple current generated by the
driver applying a voltage to the electromagnet, and a signal contained in
a signal detected by the displacement detection means other than the
displacement information of the magnetic piece is removed by subtracting
a signal obtained from an output signal of the compensation means passing
through a filter means of characteristics equivalent to the transfer
characteristics from getting current flow to the driver to current
flowing in the electromagnets and through an amplifying means for
amplifying with a specified gain, from a signal obtained from the ripple
detection means.

53. The magnetic bearing device according to claim 52, wherein the
compensation means is means to compensate a control loop phase.

54. A magnetic bearing method for rotatably supporting a magnetic piece in
a levitating state at a specified position with a plurality of
electromagnets installed opposite each other interposing the magnetic
piece therebetween comprising the steps of:supporting the magnetic piece
in a levitating state by magnetic force of the electromagnet;detecting a
displacement of the magnetic piece based on impedance change in the
electromagnet; anddriving for applying an excitation current to the
electromagnet so as to support the magnetic piece in a levitating state
at a specified position based on a detection signal obtained by the
detecting step;wherein the driving step has a pulse width modulating step
to control an excitation current of the electromagnet by modulating a
pulse width of a pulse voltage driven at a specified carrier frequency,
andthe displacement detecting step has a ripple detecting step of
detecting an amplitude of a ripple current generated by the driving
step's applying a voltage to the electromagnet, anda signal contained in
a signal detected by the displacement detecting step other than the
displacement information of the magnetic piece is removed by subtracting
a signal obtained from an output signal of the displacement detecting
step passing through a filtering step with a filter of characteristics
equivalent to the transfer characteristics from getting current flow to
the driving step to current flowing in the electromagnets and through an
amplifying step for amplifying with a specified gain, from a signal
obtained from the ripple detecting step.

55. A magnetic bearing method for rotatably supporting a magnetic piece in
a levitating state at a specified position with a plurality of
electromagnets installed opposite each other interposing the magnetic
piece therebetween comprising the steps of:supporting the magnetic piece
in a levitating state by magnetic force of the electromagnet;detecting a
displacement of the magnetic piece based on impedance change in the
electromagnet; andcompensating so as to stably support the magnetic piece
in a levitating state based on a detection signal obtained by the
detecting step; anddriving for applying an excitation current to the
electromagnet so as to support the magnetic piece in a levitating state
at a specified position based on an output signal obtained from the
compensating step;wherein the driving step has a pulse width modulating
step to control an excitation current of the electromagnet by modulating
a pulse width of a pulse voltage driven at a specified carrier
frequency,the displacement detecting step has a ripple detecting step of
detecting an amplitude of a ripple current generated by the driving
step's applying a voltage to the electromagnet, anda signal contained in
a signal detected by the displacement detecting step other than the
displacement information of the magnetic piece is removed by subtracting
a signal obtained from an output signal of the compensating step passing
through a filtering step with a filter of characteristics equivalent to
the transfer characteristics from getting current flow to the driving
step to current flowing in the electromagnets and through an amplifying
step for amplifying with a specified gain, from a signal obtained from
the ripple detecting step.

56. The magnetic bearing method according to claim 55, wherein the
compensating step is a step to compensate a control loop phase.

57. The magnetic bearing device according to claim 51, wherein the filter
means is a low-pass filter.

58. The magnetic bearing device according to claim 52, wherein the filter
means is a low-pass filter.

59. The magnetic bearing device according to claim 51, wherein the ripple
detection means has a transformer having a ripple current amplitude
detection winding for detecting an amplitude of the ripple current, and
amplifies or attenuates the amplitude of the ripple current with a
specified rate by applying the electromagnetic inductive effect of the
transformer, and outputs that amplitude as a voltage signal or a current
signal.

60. The magnetic bearing device according to claim 52, wherein the ripple
detection means has a transformer having a ripple current amplitude
detection winding for detecting an amplitude of the ripple current, and
amplifies or attenuates the amplitude of the ripple current with a
specified rate by applying the electromagnetic inductive effect of the
transformer, and outputs that amplitude as a voltage signal or a current
signal.

61. The magnetic bearing device according to claim 57, wherein the ripple
detection means has a transformer having a ripple current amplitude
detection winding for detecting an amplitude of the ripple current, and
amplifies or attenuates the amplitude of the ripple current with a
specified rate by applying the electromagnetic inductive effect of the
transformer, and outputs that amplitude as a voltage signal or a current
signal.

62. The magnetic bearing device according to claim 58, wherein the ripple
detection means has a transformer having a ripple current amplitude
detection winding for detecting an amplitude of the ripple current, and
amplifies or attenuates the amplitude of the ripple current with a
specified rate by applying the electromagnetic inductive effect of the
transformer, and outputs that amplitude as a voltage signal or a current
signal.

63. The magnetic bearing device according to claim 59, wherein a separate
winding is installed in the transformer, a signal of equal frequency to
the specified carrier frequency used in the driver is amplitude modulated
utilizing a signal obtained with the signal output from the compensation
means passing through the filter means and amplifier means, the
amplitude-modulated signal is input to the separate winding, and the
signal is subtracted by an electromagnetic induction effect from the
signal detected by the ripple current amplitude detection winding.

64. The magnetic bearing device according to claim 60, wherein a separate
winding is installed in the transformer, a signal of equal frequency to
the specified carrier frequency used in the driver is amplitude modulated
utilizing a signal obtained with the signal output from the compensation
means passing through the filter means and amplifier means, the
amplitude-modulated signal is input to the separate winding, and the
signal is subtracted by an electromagnetic induction effect from the
signal detected by the ripple current amplitude detection winding.

65. The magnetic bearing device according to claim 61, wherein a separate
winding is installed in the transformer, a signal of equal frequency to
the specified carrier frequency used in the driver is amplitude modulated
utilizing a signal obtained with the signal output from the compensation
means passing through the filter means and amplifier means, the
amplitude-modulated signal is input to the separate winding, and the
signal is subtracted by an electromagnetic induction effect from the
signal detected by the ripple current amplitude detection winding.

66. The magnetic bearing device according to claim 62, wherein a separate
winding is installed in the transformer, a signal of equal frequency to
the specified carrier frequency used in the driver is amplitude modulated
utilizing a signal obtained with the signal output from the compensation
means passing through the filter means and amplifier means, the
amplitude-modulated signal is input to the separate winding, and the
signal is subtracted by an electromagnetic induction effect from the
signal detected by the ripple current amplitude detection winding.

67. A magnetic bearing device for rotatably supporting a magnetic piece in
a levitating state at a specified position comprising:an electromagnet
for supporting the magnetic piece in a levitating state by magnetic
force;displacement detection circuit for detecting a displacement of the
magnetic piece based on impedance change in the electromagnet; anda
driver for applying an excitation current to the electromagnet so as to
support the magnetic piece in a levitating state at a specified position
based on a detection signal from the displacement detection
circuit;wherein the magnetic piece is interposed between a plurality of
electromagnets installed opposite each other,the driver is a pulse width
modulation type driver to control an excitation current of the
electromagnet by modulating a pulse width of a pulse voltage driven at a
specified carrier frequency, and includes an alternating current transfer
circuit for setting a direct current component of the excitation current
to zero, and when excitation current flows to one of the electromagnets
installed facing each other, then the alternating current transfer
circuit sets a direct current component of the excitation current of the
other electromagnet to zero.

68. A magnetic bearing device for rotatably supporting a magnetic piece in
a levitating state at a specified position comprising:an electromagnet
for supporting the magnetic piece in a levitating state by magnetic
force;displacement detection circuit for detecting a displacement of the
magnetic piece based on impedance change in the electromagnet;compensator
for compensating so as to stably support the magnetic piece in a
levitating state based on a detection signal from the displacement
detection circuit; anda driver for applying an excitation current to the
electromagnet based on an output signal from the compensator;wherein the
magnetic piece is interposed between a plurality of electromagnets
installed opposite each other,the driver is a pulse width modulation type
driver to control an excitation current of the electromagnet by
modulating a pulse width of a pulse voltage driven at a specified carrier
frequency, and includes an alternating current transfer circuit for
setting a direct current component of the excitation current to zero, and
when excitation current flows to one of the electromagnets installed
facing each other, then the alternating current transfer circuit sets a
direct current component of the excitation current of the other
electromagnet to zero.

69. The magnetic bearing device according to claim 68, wherein the
compensator is a circuit to compensate a control loop phase.

70. The magnetic bearing device according to claim 67, wherein the
alternating current transfer circuit is resonator circuit with a serially
connected coil and capacitor for electrically resonating at the same
frequency as the carrier frequency.

71. The magnetic bearing device according to claim 68, wherein the
alternating current transfer circuit is resonator circuit with a serially
connected coil and capacitor for electrically resonating at the same
frequency as the carrier frequency.

72. The magnetic bearing device according to claim 67, comprising a
subtractor which detects respective excitation currents flowing in the
electromagnets installed opposite each other sandwiching the magnetic
piece and subtracts a deference of signals of the detected currents after
the deference signal passing through a filter of specific characteristics
from the output of the displacement detection circuit, wherein the
specific characteristics are equivalent to the frequency characteristics
of the displacement detection circuit.

73. The magnetic bearing device according to claim 68, comprising a
subtractor which detects respective excitation currents flowing in the
electromagnets installed opposite each other sandwiching the magnetic
piece and subtracts a deference of signals of the detected currents after
the deference signal passing through a filter of specific characteristics
from the output of the displacement detection circuit, wherein the
specific characteristics are equivalent to the frequency characteristics
of the displacement detection circuit.

74. The magnetic bearing device according to claim 67, wherein signals
contained in signals detected by the displacement detection circuit other
than the displacement information of the magnetic piece are removed by AM
modulating the carrier frequency signal with an output signal of a filter
of characteristics equivalent to the transfer characteristics from
getting current flow to the driver to current flowing in the
electromagnets and by subtracting a signal obtained by the AM modulation
from a signal obtained from comparing a value of a ripple current
occurring after applying a pulse voltage to an electromagnets in which
excitation current is flowing, among an electromagnet pair installed
facing each other, with a value of a ripple current occurring after
applying a voltage via the alternating current transfer circuit to the
other magnet whose direct current component within the excitation current
is set to zero.

75. The magnetic bearing device according to claim 68, wherein signals
contained in signals detected by the displacement detection circuit other
than the displacement information of the magnetic piece are removed by AM
modulating the carrier frequency signal with an output signal of a filter
of characteristics equivalent to the transfer characteristics from
getting current flow to the driver to current flowing in the
electromagnets and by subtracting a signal obtained by the AM modulation
from a signal obtained from comparing a value of a ripple current
occurring after applying a pulse voltage to an electromagnets in which
excitation current is flowing, among an electromagnet pair installed
facing each other, with a value of a ripple current occurring after
applying a voltage via the alternating current transfer circuit to the
other magnet whose direct current component within the excitation current
is set to zero.

76. The magnetic bearing device according to claim 67, further comprising
a linearization circuit for changing the non-linear relation between the
magnetic force of the electromagnet exerted on the magnetic piece and the
excitation current into a linear relation.

77. The magnetic bearing device according to claim 68, further comprising
a linearization circuit for changing the non-linear relation between the
magnetic force of the electromagnet exerted on the magnetic piece and the
excitation current into a linear relation.

78. The magnetic bearing device according to claim 76, wherein the
compensator, or the linearization circuit, or the compensator and the
linearization circuit acquire an output by digital processing.

79. The magnetic bearing device according to claim 77, wherein the
compensator, or the linearization circuit, or the compensator and the
linearization circuit acquire an output by digital processing.

80. A magnetic bearing device for rotatably supporting a magnetic piece in
a levitating state at a specified position comprising:an electromagnet
for supporting the magnetic piece in a levitating state by magnetic
force;a displacement detection circuit for detecting a displacement of
the magnetic piece based on impedance change in the electromagnet; anda
driver for applying an excitation current to the electromagnet so as to
support the magnetic piece in a levitating state at a specified position
based on a detection signal from the displacement detection
circuit;wherein the driver is a pulse width modulation type driver to
control an excitation current of the electromagnet by modulating a pulse
width of a pulse voltage driven at a specified carrier frequency, andthe
displacement detection circuit has a ripple detection circuit for
detecting an amplitude of a ripple current generated by the driver
applying a voltage to the electromagnet, and a signal contained in a
signal detected by the displacement detection circuit other than the
displacement information of the magnetic piece is removed by subtracting
a signal obtained from an output signal of the displacement detection
circuit passing through a filter of characteristics equivalent to the
transfer characteristics from getting current flow to the driver to
current flowing in the electromagnets and through an amplifying means for
amplifying with a specified gain, from a signal obtained from the ripple
detection means.

81. A magnetic bearing device for rotatably supporting a magnetic piece in
a levitating state at a specified position comprising:an electromagnet
for supporting the magnetic piece in a levitating state by magnetic
force;a displacement detection circuit for detecting a displacement of
the magnetic piece based on impedance change in the electromagnet;a
compensator for compensating so as to stably support the magnetic piece
in a levitating state based on a detection output signal from the
displacement detection circuit; anda driver for applying an excitation
current to the electromagnet based on an output signal from the
compensator;wherein the driver is a pulse width modulation type driver to
control an excitation current of the electromagnet by modulating a pulse
width of a pulse voltage driven at a specified carrier frequency, andthe
displacement detection circuit has a ripple detection circuit for
detecting an amplitude of a ripple current generated by the driver
applying a voltage to the electromagnet, and a signal contained in a
signal detected by the displacement detection circuit other than the
displacement information of the magnetic piece is removed by subtracting
a signal obtained from an output signal of the compensator passing
through a filter of characteristics equivalent to the transfer
characteristics from getting current flow to the driver to current
flowing in the electromagnets and through an amplifying means for
amplifying with a specified gain, from a signal obtained from the ripple
detection circuit.

82. The magnetic bearing device according to claim 81, wherein the
compensator is a circuit to compensate a control loop phase.

83. The magnetic bearing device according to claim 80, wherein the filter
is a low-pass filter.

84. The magnetic bearing device according to claim 81, wherein the filter
is a low-pass filter.

85. The magnetic bearing device according to claim 80, wherein the ripple
detection circuit has a transformer having a ripple current amplitude
detection winding for detecting an amplitude of the ripple current, and
amplifies or attenuates the amplitude of the ripple current with a
specified rate by applying the electromagnetic inductive effect of the
transformer, and outputs that amplitude as a voltage signal or a current
signal.

86. The magnetic bearing device according to claim 81, wherein the ripple
detection circuit has a transformer having a ripple current amplitude
detection winding for detecting an amplitude of the ripple current, and
amplifies or attenuates the amplitude of the ripple current with a
specified rate by applying the electromagnetic inductive effect of the
transformer, and outputs that amplitude as a voltage signal or a current
signal.

87. The magnetic bearing device according to claim 85, wherein a separate
winding is installed in the transformer, a signal of equal frequency to
the specified carrier frequency used in the driver is amplitude-modulated
utilizing a signal obtained with the signal output from the compensator
passing through the filter and the amplifier, the amplitude-modulated
signal is input to the separate winding, and the signal is subtracted by
an electromagnetic induction effect from the signal detected by the
ripple current amplitude detection winding.

88. The magnetic bearing device according to claim 86, wherein a separate
winding is installed in the transformer, a signal of equal frequency to
the specified carrier frequency used in the driver is amplitude-modulated
utilizing a signal obtained with the signal output from the compensator
passing through the filter means the amplifier, the amplitude-modulated
signal is input to the separate winding, and the signal is subtracted by
an electromagnetic induction effect from the signal detected by the
ripple current amplitude detection winding.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a magnetic bearing device and a
magnetic bearing method for detecting the displacement of a magnetic
piece by the change in impedance of the electromagnet for supporting the
magnetic piece in a levitating state, and controlling the magnetic force
of that electromagnet to support the magnetic piece in a specified
position, and more particularly to a magnetic bearing device and a
magnetic bearing method suitable for energy-saving.

[0002]The present invention further relates in particular to a magnetic
bearing device and a magnetic bearing method including a displacement
detection means for detecting with high accuracy the displacement of a
magnetic piece by the change in impedance of the electromagnet.

BACKGROUND ART

[0003]Magnetic bearing devices utilizing magnetic force for non-contact
support of a rotating piece are widely used in rotating equipment in the
Background Art such as turbo molecular pumps that require a rotating
piece be rotated at high-speed. Positive features of the magnetic bearing
device are that it lowers the rotation resistance of the rotating piece
supported by the bearing, generates no particles due to wear, and
requires none of the maintenance usually needed due to bearing wear, and
no contamination occurs from lubricant fluid in the bearing, etc.

[0004]Demands have increased in recent years for magnetic bearing devices
that offer a lower cost, more space-saving and higher-speed rotation,
etc. The technology of the Background Art has employed sensor-less
magnetic bearings that did not require a displacement sensor. Instead of
a displacement sensor, sensor-less magnetic bearings has utilized the
change in impedance of the electromagnet as one method for detecting
displacement of a rotating piece.

[0005]Most of the impedance of an electromagnet is made up of the
inductance component. The change in this inductance is utilized to detect
displacement of the rotating piece. The shape, number of windings, and
material of the electromagnet core, as well as the gap between the
rotating piece and electromagnet are the main factors in determining the
inductance of the electromagnet. The material of the electromagnet core,
shape, and the number of windings, are determined in the electromagnet
design stage. The change in the inductance of the electromagnet occurs
due to a change in the gap between the electromagnet and the rotating
piece. In other words, the inductance of the electromagnet changes due to
displacement of the rotating piece, and the displacement of that rotating
piece can be detected by measuring this change. By feeding back this
acquired displacement signal, the rotating piece can be supported in a
non-contact levitating state at a specified position.

[0006]A non-linear relation is generally established between the magnetic
force exerted on the rotating piece and the excitation current of the
electromagnet. In the Background Art, a pair of electromagnets are
therefore installed facing each other to sandwich the rotating piece, and
by then applying a specified direct current bias to each of the opposing
electromagnets, a linear relation can be established between the magnetic
force exerted on the rotating piece and the excitation current of the
electromagnet so that the rotating piece is stably supported in a
levitating state as a simple linear system (see Patent Document 1 e.g.).

[0007]The method of the Background Art also had the following problems.
When there is an actual change in the electromagnet current, then the
magnetic characteristics of the electromagnet core change. The inductance
of the electromagnet therefore changes even if there is no displacement
of the rotating piece. Errors therefore occur when detecting
displacement, due to this change in inductance caused by the current of
the electromagnet. When applying an external force to the rotating piece
via the electromagnets in general, the rotating piece displacement is
large for the force at low frequencies, and the rotating piece
displacement is small for the force at high frequencies. The change in
inductance at low frequencies therefore causes a larger change in
rotating piece displacement than from changes due to the electromagnet
current, and there is little effect from displacement detection errors
induced by the electromagnet current. However, the change in inductance
at high frequencies renders the opposite effect in case that changes due
to the electromagnet current are larger, and the effect to displacement
detection errors is large. Therefore, control of the magnetic bearing
tended to be unstable in the high frequency range.

[0008]As one countermeasure to this problem of unstable magnetic bearing
control in the high frequency range, the Background Art as shown in the
Patent Document 1 employed a method to detect the electromagnet current
and eliminate the differential in predicted displacement detection error
from the displacement detection signal that was detected via the change
in impedance of the electromagnets (Patent document 1 e.g.).

[Patent Document 1] Laid Open Patent Application 2004-132537

DISCLOSURE OF INVENTION

Problems to be Solved

[0009]However, forming a linear relation between the excitation current in
the electromagnet and the magnetic force exerted on the rotating piece by
applying a DC bias to the electromagnet, constantly generates a copper
loss in the electromagnet coil. This copper loss generates redundant heat
in the magnetic bearing and therefore causes energy consumption to
increase.

[0010]The sensor-less magnetic bearing of the Background Art detects
displacement of the rotating piece by measuring the change in the
electromagnet inductance. Here, in order to find the change in
inductance, the magnetic bearing detects the amplitude of the ripple
current generated by the PWM voltage applied to the electromagnet. A
driver for generating the PWM voltage applies a PWM voltage to the
electromagnet when in an ON state, and when in an OFF state returns the
energy accumulated while the electromagnet is ON, to the PWM power supply
via a flywheel diode. This driver utilizes a flywheel diode and so can
only allow current flow to the electromagnet in one direction, and no
ripple current is generated in the vicinity of the region where the
electromagnet current is zero so detecting the rotating piece
displacement is impossible. The magnetic bearing of the Background Art
therefore constantly required a current flow larger than zero in the
opposing electromagnet. This current flow did not usually pose a problem
since a bias current was flowing to make a linear relationship between
the magnetic force and the electrical current, in the magnetic bearing.
However, the Background Art failed to provide a sensor-less magnetic
bearing with zero bias magnetic bearing of a method of eliminating the
flow of a bias current in the electromagnet to improve power consumption
and to lower heat generation.

[0011]In view of the above points, the present invention has the first
object of providing a sensor-less magnetic bearing device and a magnetic
bearing method that do not require a bias current for making the relation
between the excitation current and magnetic force of the electromagnet
linear to save energy.

[0012]This method of the Background Art as shown Patent Document 1 above
required a filter with characteristics equivalent to the displacement
detection means frequency characteristics in order to predict the
differential in the displacement detection error. However, the frequency
characteristic of the displacement detection means is of a higher order,
so that contriving a filter with characteristics equivalent to the
displacement detection means characteristics was difficult. Consequently,
eliminating with high precision the displacement detection error was
difficult especially in the high frequency range.

[0013]The present invention therefore has the second object of eliminating
the above mentioned problems by providing a magnetic bearing device and a
magnetic bearing method capable of detecting displacement of the rotating
piece in the high frequency range with high precision, and achieving
stable magnetic bearing control up to the high frequency range.

Measures for Solving the Problems

[0014]The aspect (1) of the invention for solving the above mentioned
problem is a magnetic bearing device for rotatably supporting a magnetic
piece in a levitating state at a specified position comprising, an
electromagnet for supporting the magnetic piece in a levitating state by
magnetic force, displacement detection means for detecting a displacement
of the magnetic piece based on impedance change in the electromagnet,
compensation means for compensating so as to stably support the magnetic
piece in a levitating state based on a detection signal from the
displacement detection means, and a driver for applying an excitation
current to the electromagnet based on an output signal from the
compensation means, wherein the magnetic piece is interposed between a
plurality of electromagnets installed opposite each other, the driver is
a pulse width modulation type driver to control an excitation current of
the electromagnet by modulating a pulse width of a pulse voltage driven
at a specified carrier frequency, and includes an alternating current
transfer means for setting a direct current component of the excitation
current to zero, and when excitation current flows to one of the
electromagnets installed facing each other, then the alternating current
transfer means sets a direct current component of the excitation current
of the other electromagnet to zero.

[0015]The aspect (2) of the invention is the above mentioned magnetic
bearing device, wherein the alternating current transfer means is
preferably resonator means with a serially connected coil and capacitor
for electrically resonating at the same frequency as the carrier
frequency.

[0016]The aspect (3) of the invention is the above mentioned magnetic
bearing device which preferably detects respective excitation currents
flowing in the electromagnets installed opposite each other sandwiching
the magnetic piece and subtracts a deference of signals of the detected
currents after the deference signal passing through filter means of the
equivalent characteristics to transfer characteristics of the
displacement detection means and an amplifier for amplifying with a
specified gain, from the output signal of the displacement detecting
means, wherein the specific frequency characteristics are equivalent to
the transfer characteristics of the displacement detection means, and
thus a signal contained in the detected signal other than the
displacement information of the magnetic piece is removed.

[0017]The aspect (4) of the invention is the above mentioned magnetic
bearing device, wherein preferably the compensation means removes a
signal contained in a signal detected by the displacement detection means
other than the displacement information of the magnetic piece, by
subtracting a signal obtained after the signal passing through filter of
the equivalent characteristics to the characteristics from getting
current flow to the driver to current flowing in the electromagnets and
an amplifier for amplifying with a specified gain, from a signal obtained
from comparing a value of a ripple current occurring after applying the
pulse voltage to an electromagnet in which excitation current is flowing,
among the pair of electromagnets installed facing each other, with a
value of a ripple current occurring after applying a voltage via the
resonator means to the other magnet whose direct current component within
the excitation current is set to zero.

[0018]The aspect (5) of the invention is the above mentioned magnetic
bearing device preferably further comprising a linearization means for
changing the non-linear relation between the magnetic force of the
electromagnet exerted on the magnetic piece and the excitation current
into a linear relation.

[0019]The aspect (6) of the invention is the above mentioned magnetic
bearing device, wherein the compensation means, or the linearization
means, or the compensation means and the linearization means preferably
acquire an output by digital processing.

[0020]The aspect (7) of the invention for solving the above mentioned
problem is a magnetic bearing device for rotatably supporting a magnetic
piece in a levitating state at a specified position comprising, an
electromagnet for supporting the magnetic piece in a levitating state by
magnetic force, a displacement detection means for detecting a
displacement of the magnetic piece based on impedance change in the
electromagnet, a compensation means for compensating so as to stably
support the magnetic piece in a levitating state based on a detection
output signal from the displacement detection means, and a driver for
applying an excitation current to the electromagnet based on an output
signal from the compensation means, wherein the driver is a pulse width
modulation type driver to control an excitation current of the
electromagnet by modulating a pulse width of a pulse voltage driven at a
specified carrier frequency, and the displacement detection means has a
ripple detection means for detecting an amplitude of a ripple current
generated by the driver applying a voltage to the electromagnet, and a
signal contained in a signal detected by the displacement detection means
other than the displacement information of the magnetic piece is removed
by subtracting a signal obtained from an output signal of the
compensation means passing through a filter means of characteristics
equivalent to the transfer characteristics from getting current flow to
the driver to current flowing in the electromagnets and through an
amplifying means for amplifying with a specified gain, from a signal
obtained from the ripple detection means.

[0021]The aspect (8) of the invention is the above mentioned magnetic
bearing device, wherein preferably the filter means is a low-pass filter.

[0022]The aspect (9) of the invention is the above mentioned magnetic
bearing device, wherein preferably the ripple detection means has a
transformer having a ripple current amplitude detection winding for
detecting an amplitude of the ripple current, and amplifies or attenuates
the amplitude of the ripple with a specified rate by applying the
electromagnetic inductive effect of the transformer, and outputs that
amplitude as a voltage signal or a current signal.

[0023]The aspect (10) of the invention is the above mentioned magnetic
bearing device, wherein a separate winding is installed in the
transformer, and a signal output from the compensation means passes
through the filter means and the amplifying means, a signal obtained by
the passing through is amplitude-modulated by utilizing a frequency equal
to the specified carrier frequency used in the driver and the
amplitude-modulated signal is input to the separate winding, and the
signal is subtracted by an electromagnetic induction effect from the
signal detected by the ripple current amplitude detection winding.

Effect of the Invention

[0024]In the aspect (1) of the present invention, no bias current flows in
the electromagnet so there is no need for a circuit to supply a bias
current. Therefore, along with lowering costs there is also the advantage
that the electromagnet coil wastes no redundant energy so an
energy-saving magnetic bearing device can be provided. The magnetic force
is applied to the magnetic piece from either one of the opposing magnets
so the magnetic force applied to the magnetic piece can be easily
estimated. The excitation current only flows in either one of the
opposing magnets so energy can be saved. Moreover, a ripple current is
generated by applying an alternating voltage oscillating at the same
frequency as the carrier frequency of the PWM driver. The change in
inductance of the electromagnet can be detected from this ripple current
even if the electromagnet excitation current is near zero in a DC
component, so that the displacement of the magnetic piece can be detected
from this change in inductance.

[0025]In another aspect (2) of the present invention, the alternating
current transfer means is composed of a coil and capacitor connected in
series, and can therefore be easily constructed using a commercially
available coil and capacitor.

[0026]In yet another aspect (3) of the present invention, the displacement
detection error caused by the effect of a change in magnetic properties
due to the change of the excitation current in the electromagnet, in
other words a change of inductance of the magnet due to the excitation
current, can be estimated from the electromagnet's excitation current.
The displacement can therefore be detected with good accuracy by removing
the error differential from the displacement detection signal.

[0027]In still another aspect (4) of the present invention, the
displacement detection error caused by the effect of a change in magnetic
properties due to the change of the excitation current in the
electromagnet, in other words a change of inductance of the magnet due to
the excitation current, can be estimated from a signal output from a
compensation means. The displacement can therefore be detected with good
accuracy by removing the error differential from the displacement
detection signal.

[0028]In a further aspect (5) of the present invention, the magnetic
bearing device includes a linearization means for making the relation
between the magnetic force exerted on the magnetic piece and the
excitation current of the electromagnet linear. An electrical current
control signal can therefore be sent to the driver to make the magnetic
force applied to the magnetic piece linear versus the displacement signal
obtained from the displacement detection means so that a simple linear
system can be established for the magnetic bearing device.

[0029]In a yet further aspect (6) of the present invention, the
compensation means, or the linearization means, or the compensation means
and linearization means are structured to acquire an output by digital
processing. The characteristics of the compensation means and
linearization means can therefore be easily programmed via a digital
processing means such as DSP (Digital Signal Processor) to eliminate the
bothersome task of building-and-assembling circuits and soldering work
and to also allow those characteristics to easily change.

[0030]In a still further aspect (7) of the present invention, by
subtracting the signal obtained after passing through a filter means
containing characteristics equivalent to the transfer characteristics
from the compensation means output signal commanding the driver to flow
current in the electromagnet to the flow of current in the electromagnet
and through an amplifier means for amplifying the signal by a specified
gain, from the signal obtained from the ripple detection means, signals
other than for the rotating piece displacement information contained in
the signal detected by the displacement detection means can be removed.
Therefore the transfer characteristics between the driver and the
electromagnets are low order characteristics and a filter means with
transfer characteristics equivalent to the same can easily be realized so
that the displacement detection error due to the electromagnet current
can be removed with good accuracy up through the high frequency range,
and a magnetic bearing device capable of stable magnetic bearing control
through the high frequency range can be provided.

[0031]In another aspect (8) of the present invention, the inductance
characteristics of the electromagnet (frequency characteristics from the
driver to electromagnet) basically make up the low-pass filter
characteristics, so the filter means can be constituted as a low pass
filter to remove the error of the displacement detected with the
electromagnet current, up through the high frequency region.

[0032]In still another aspect (9) of the present invention, the ripple
detection means includes a transformer, and by insulating the
transformer, the amplitude signal of the ripple current as the
displacement information contained in the electromagnet current can be
applied to the low-voltage circuit as it is even for a high-voltage
driver. Moreover a resonance circuit with a capacitor and winding can be
constructed on the transformer output by connecting a capacitor or a
capacitor and resistor in parallel across the winding terminals on the
transformer output, so that a filter for removing any frequencies other
than those containing the displacement information can be realized.

[0033]In yet another aspect (10) of the present invention, a separate
winding is installed in the transformer, and a frequency signal identical
to the specified carrier frequency used in the driver is
amplitude-modulated utilizing a signal obtained with the signal output
from the compensation means passing through the filter means and
amplifier means, and the AM (amplitude-modulated) signal is input in this
separate winding, and its signal is subtracted by an electromagnetic
induction effect from the signal detected by the ripple current amplitude
detecting winding, so that the predicted displacement detection error is
in this way subtracted via the transformer from the displacement
detection signal. This aspect of the present invention requires no
subtraction means such as a new processing circuit, so that the cost can
be reduced.

[0034]This application is based on the Patent Applications No. 2005-196635
filed on Jul. 5, 2005 and 2005-196636 filed on Jul. 5, 2005 in Japan, the
contents of which are hereby incorporated in its entirety by reference
into the present application, as part thereof.

[0035]The present invention will become more fully understood from the
detailed description given hereinbelow. However, the detailed description
and the specific embodiment are illustrated of desired embodiments of the
present invention and are described only for the purpose of explanation.
Various changes and modifications will be apparent to those ordinary
skilled in the art on the basis of the detailed description.

[0036]The applicant has no intention to give to public any disclosed
embodiment. Among the disclosed changes and modifications, those which
may not literally fall within the scope of the patent claims constitute,
therefore, a part of the present invention in the sense of doctrine of
equivalents.

[0037]The use of the terms "a" and "an" and "the" and similar referents in
the context of describing the invention (especially in the context of the
following claims) are to be construed to cover both the singular and the
plural, unless otherwise indicated herein or clearly contradicted by
context. The use of any and all examples, or exemplary language (e.g.,
"such as") provided herein, is intended merely to better illuminate the
invention and does not pose a limitation on the scope of the invention
unless otherwise claimed.

BRIEF DESCRIPTION OF DRAWINGS

[0038]FIG. 1 is a block diagram showing the configuration of the magnetic
bearing device of the first embodiment of the present invention;

[0039]FIG. 2 is a drawing showing the configuration of the current supply
section of the driver for the magnetic bearing device of the first
embodiment of the present invention;

[0040]FIG. 3 is a drawing showing the configuration of the control section
for the driver of the magnetic bearing device of the first embodiment of
the present invention;

[0041]FIG. 4 is a drawing showing the signal of each section of the
magnetic bearing device of the first embodiment of the present invention;

[0042]FIG. 5 is a drawing showing the switching timing modes for switching
element in the control section of the driver in the magnetic bearing
device of the first embodiment of the present invention;

[0043]FIG. 6A is a drawing for describing the operation of the magnetic
bearing device of the first embodiment of the present invention (at time
T1);

[0044]FIG. 6B is a drawing for describing the operation of the magnetic
bearing device of the first embodiment of the present invention (at time
T2);

[0045]FIG. 7A is a drawing for describing the operation of the magnetic
bearing device of the first embodiment of the present invention (at time
T3);

[0046]FIG. 7B is a drawing for describing the operation of the magnetic
bearing device of the first embodiment of the present invention (at time
T4);

[0047]FIG. 8A is a drawing for describing the operation of the magnetic
bearing device of the first embodiment of the present invention (at time
T5);

[0048]FIG. 8B is a drawing for describing the operation of the magnetic
bearing device of the first embodiment of the present invention (at time
T6);

[0049]FIG. 9A is a drawing for describing the operation of the magnetic
bearing device of the first embodiment of the present invention (at time
T7);

[0050]FIG. 9B is a drawing for describing the operation of the magnetic
bearing device of the first embodiment of the present invention (at time
T8);

[0051]FIG. 10 is a diagram showing an example of the configuration of the
magnetic bearing device of the second embodiment of the present
invention;

[0052]FIG. 11 is a drawing showing the signal waveforms from each unit of
the magnetic bearing device of the present invention;

[0053]FIG. 12 is a drawing showing the relation of the excitation current
to the inductance of the electromagnet of this magnetic bearing device;

[0054]FIG. 13 is a block diagram showing the first example of
configuration where the detection error due to the control current is
removed;

[0055]FIG. 14 is a block diagram showing the second example of
configuration where the detection error due to the control current is
removed;

[0056]FIG. 15 is a block diagram showing the third example of
configuration where the detection error due to the control current is
removed;

[0057]FIG. 16 is a block diagram showing the fourth example of
configuration where the detection error due to the control current is
removed; and

[0058]FIG. 17 is a block diagram showing the configuration of the driver
108.

[0116]The first embodiment of the present invention is described next with
reference to FIG. 1 through FIG. 4 and FIG. 13 through FIG. 16. FIG. 1 is
a block diagram showing the configuration of the magnetic bearing device
of the first embodiment of the present invention. FIG. 2 is a drawing
showing the configuration of the current supply section of the driver.
FIG. 3 is a drawing showing the configuration of the control section for
the driver. FIG. 4 is a drawing showing the signal of each section of the
magnetic bearing device. That is, FIG. 4 is a drawing showing the signal
of each section of the magnetic bearing device when the control signal u'
is input to the driver 204, and the duty ratio of the current to the
magnet 2 and the excitation current tend to be larger toward the left
hand side in the chart. They are opposite toward the right hand side.

[0117]As can be seen in the figures, the magnetic bearing device includes
a rotor 1, an electromagnet 2, and electromagnet 3, a driver 204, a
band-pass filter (BPF) 205, a demodulation means 206, a compensation
means 207, a linearization means 208, a digital processing means 209, an
oscillator 210, an adder 211, and a comparator 212. The rotor 1 or in
other words the rotating piece constituted with a magnetic piece is
interposed between a pair of electromagnets 2, 3 on opposite sides to
support the rotor 1 in a non-contact, levitating state. These
electromagnets 2, 3 support rotor 1 in some degrees of freedom. Usually
multiple pairs of magnets are preferably utilized to support the rotor in
a levitating state in 5 degrees of freedom in directions other than rotor
axial rotation direction. However, for purposes of simplicity, only 1
degree of freedom is described here. The motor for rotating the rotor 1
is also omitted, and only the magnetic bearing is described. Here, the
compensation means 207 is a well known feedback control system, which is
a loop phase compensation system to keep the control stable.

[0119]The driver 204 is a PWM type driver for supplying an excitation
current to the electromagnets 2, 3. This driver 204 supplies excitation
currents i1, i2 to the electromagnets 2, 3 by applying PWM
voltages e1, e2 made up of the frequency fc generated by
oscillator 210 as a carrier frequency. The resonator means 225 is
constructed with a coil Ls and a capacitor Cs connected serially. The
resonator means 225 is set to resonate at a frequency identical to
carrier frequency fc. In other words, the impedance of the resonator
means 225 is approximately zero at the frequency fc. The carrier
frequency fc used in this magnetic bearing device is equal to 30 kHz, but
frequencies from 10 kHz to 100 kHz may be utilized as the carrier
frequency fc.

[0120]The driver 204 feeds a control current ic to the electromagnets 2, 3
by controlling the duty ratio of PWM voltages e1, e2 based on
the control signal u', to generate the desired magnetic force. Applying
the PWM voltages e1, e2 to electromagnets 2, 3 allows the
ripple currents ir1, ir2 to flow while multiplexed with a
control current ic. In other words, the current containing the control
current ic and ripple currents ir1, ir2 flows in the
electromagnets 2, 3, where the current ic directly contributes to the
magnetic levitation and the currents ir1, ir2 do not directly
contribute to the magnetic levitation but have displacement information
about the rotor 1. As shown in FIG. 5, the switching devices SW1,
SW2, SW3, SW4, turn on and off in four patterns of the
Mode I-1, Mode I-2, Mode II-1 and Mode II-2. In other words, in Mode I-1,
the switching device SW1 is on, the SW2 is off, the SW3 is
off, and the SW4 is on. In Mode I-2, the switching device SW1
is off, the SW2 is off, the SW3 is on, and the SW4 is off.
In Mode II-1, the switching device SW1 is off, the SW2 is on,
the SW3 is on, and the SW4 is off. In Mode II-2, the switching
device SW1 is off, the SW2 is off, the SW3 is off, and the
SW4 is on.

[0121]FIG. 6 through FIG. 9 are drawings for describing the operation of
the magnetic bearing device shown in the block diagram of FIG. 1, at the
times T1 through T8 in FIG. 4. FIG. 6A shows the operation at
time T1 and FIG. 6B shows the operation at time T2 at the
switch timing in Mode I-2 respectively. FIG. 7A shows the operation at
time T3, and FIG. 7B shows the operation at time T4 at the
switch timing in Mode I-1 respectively. FIG. 8A shows the operation at
time T5, and FIG. 8B shows the operation at time T6 at the
switch timing in Mode II-1 respectively. FIG. 9A shows the operation at
time T7, and FIG. 9B shows the operation at time T8 at the
switch timing in Mode II-2 respectively.

[0122]As shown in FIG. 6A, in switch timing Mode I-2 at time T1
(excitation current i1: positive, excitation current i2:
negative), the switching device SW3 is on, and the switching devices
SW1, SW2, SW4, are off, so that the excitation current
i1 flows to the electromagnet 2, and the excitation current i2
flows via the resonator means 225 to the electromagnet 3. At time T2
(excitation current i1: positive, excitation current i2:
positive, i1>i2), the excitation current i1 flows to
the electromagnet 2, and the excitation current i2 flows via the
resonator means 225 to the electromagnet 3.

[0123]As shown in FIG. 7A, in switch timing Mode I-1 at time T3
(excitation current i1: positive, excitation current i2:
positive, i1>i2), the switching devices SW1 and
SW4 are on and the switching devices SW2 and SW3 are off,
so that an excitation current i1 is applied to the electromagnet 2,
and an excitation current i2 flows via the resonator means 225 to
the electromagnet 3. Also, as shown in FIG. 7B, at time T4
(excitation current i1: positive, excitation current i2:
negative) the excitation current i1 flows to the electromagnet 2,
and the excitation current i2 flows via the resonator means 225 to
the electromagnet 3.

[0124]As shown in FIG. 8A, in switch timing Mode II-1 at time T5
(i1: positive, i2: positive, i1>i2), the switching
devices SW2 and SW3 are on, and the switching devices SW1
and SW4 are off, so that the excitation current i2 flows to the
electromagnet 3, and the excitation current i1 flows via the
resonator means 225 to the electromagnet 2. Also as shown in FIG. 8B, at
time T6 (excitation current i1: negative, excitation current
i2: positive), the excitation current i2 flows to the
electromagnet 3 and the excitation current i1 flows via the
resonator means 225 to the electromagnet 2.

[0125]As shown in FIG. 9A, in switch timing Mode II-2 at time T7
(excitation current i1: negative, excitation current i2:
positive), the switching device SW4 is on, and the switching devices
SW1, SW2, SW3, are off so that the excitation current
i2 flows to the electromagnet 3, and the excitation current i1
flows via the resonator means 225 to the electromagnet 2. Also as shown
in FIG. 9B, at time T8, (excitation current i1: positive,
excitation current i2: positive, i1<i2), the excitation
current i2 flows to the electromagnet 3 and the excitation current
i1 flows via the resonator means 225 to the electromagnet 2.

[0126]In the switch timings in Mode I-1 and Mode I-2, a PWM voltage
e1 is applied to the electromagnet 2, and the desired control
current ic is supplied by controlling the duty of PWM voltage e1. A
PWM voltage e2 is applied via the resonator means 225 to the
electromagnet 3 during this time, and the DC component removed from the
current that is flowing. The PWM voltage e1 and e2 are applied
as mutually reverse voltages at this time, and the ripple currents
ir1 and ir2 oscillate in mutually reverse directions as shown
in FIG. 4. Therefore, at the switch timings of Mode I-1 and Mode I-2, the
excitation currents i1 and i2 are as shown by the following
formula (1).

i1=ic+ir1

i2=-ir2 (1)

[0127]In the switch timing in Mode II-1 and Mode II-2 on the other hand,
the control current ic flows to the electromagnet 3, opposite to the case
in Mode I-1 and Mode I-2. Therefore at the switch timings Mode II-1 and
Mode II-2, the excitation currents i1 and i2 are as shown by
the following formula (2).

i1=ir1

i2=ic-ir2 (2)

[0128]As shown in FIG. 4, if the control current ic is flowing in either
one of the electromagnets 2, 3, then the driver 204 can set the other
electromagnet to only allow ripple current flow whose time averaged
current is zero. The switch signals S1, S2, S3, S4
switch the respective switching devices SW1, SW2, SW3,
SW4, on/off. The switching devices SW1, SW2, SW3,
SW4 turn on when the switch signals S1, S2, S3,
S4 are Hi (high level), and turn off when they are Lo (low level).

[0129]Expressing the excitation current i1 and i2 of formulas
(1) and (2), respectively as i1' and i2' with their
high-frequency component removed by a low-pass filter yields the formulas
(1') and (2'). The i1' and i2' become only the control current
information ic or zero because the ripple currents ir1 and ir2
components of currents i1 and i2 are removed.

i1'=ic

i2'=0 (1')

i1'=0

i2'=ic (2')

[0130]As shown in FIG. 3, the driver 204 detects the i1' and
i2', the currents i1' and i2' are mutually subtracted in
the subtractor 235, the output signal from the subtractor 235 is fed back
and compared with the control signal u' in the comparator 236 and the
compared signal is amplified by the amplifier 232 with gain of K. At the
comparator 237, the signal output from the amplifier 232 is compared with
a triangular waveform signal St of a frequency equal to the PWM carrier
frequency fc output from the oscillator 210, whereby the PWM signal Sp of
the desired duty ratio is obtained. The switch signal S4 and the
switch signal S3 inverted by the inverter 229 are output via the PWM
signal Sp so that the on and off timing of switching devices SW3 and
SW4 always operate opposite each other. Thus, the control signal
ic is obtained which is corresponding to the control signal u'.

[0131]The comparator 233 contains a tiny comparison reference value
TH1 (See FIG. 4) set at a value equal to or larger than the
amplitude of the ripple current ir1. The comparator 233 outputs a Lo
(low) when the control current i1' of electromagnet 2 is larger than
the comparator reference value TH1, and outputs a Hi (high) when
equal to or smaller than the comparison reference value TH1. The
comparator 234 contains a tiny comparison reference value TH2 (See
FIG. 4) set at a value equal to or larger than the ripple current
ir2 the same as for comparator 233. The comparator 234 outputs a Lo
(low) when the control current i2' of electromagnet 3 is larger than
the comparator reference value TH2, and outputs a Hi (high) when
equal to or smaller than the comparison reference value TH2. These
comparison reference values TH1 and TH2 are set so that a
current definitely flows in either the i1' or i2'. These
comparison reference values TH1 and TH2 may be set to the same
value. The switch signal S2 may be combined with an AND processor
230 (See FIG. 3) to support a Lo output when the control current i1'
of electromagnet 2 is larger than TH1, as shown in FIG. 4. When the
control current i1' is equal to or smaller than TH1, the switch
signal S2 may be synchronized with the switching device SW3 to
turn the switching device SW2 on/off.

[0132]The switch signal S1 may be combined with an AND processor 231
(See FIG. 3) to support a Lo output when the control current i2' of
electromagnet 3 is larger than TH2. When the control current
i2' is equal to or smaller than TH2, the switch signal S1
may be synchronized with the switching device SW4 to turn the
switching device SW1 on/off. In the period here where the switching
device SW1 is off, current is always supplied via the resonator
means 225 to the electromagnet 2, the direct current component is
removed, only the ripple ir1 current flows, so that the
time-averaged current becomes zero. Conversely in the period where the
switching device SW2 is off, the time-averaged current of the
electromagnet 3 becomes zero, and only the ripple current ir2 flows.

[0133]As shown in FIG. 4, the driver 204 in this way controls operation,
so that if the control current ic flows in either one of the
electromagnets 2, 3, then the time-averaged current flowing in the other
electromagnet becomes zero. Moreover, by feeding back the differential
between the i1' and i2' values, the control current can excite
the electromagnet 2 and electromagnet 3 based on the control signal u'.
The control current ic can here flow to the electromagnet 2 if the
control signal u' is positive. The control current ic can here flow to
the electromagnet 3 if the control signal u' is negative.

[0134]When the PWM voltages e1, e2 applied to the electromagnets
2, 3, a current then flows according to the respective impedance, and the
PWM voltages e1, e2 are at a high frequency so that the
impedance components of the electromagnets 2, 3 may be considered the
inductance component. Summing the excitation current i1 and the
excitation current i2 per formulas (1) and (2) yields the following
formula (3).

i1+i2=ic+(ir1-ir2) (3)

[0135]The ir1-ir2 in formula (3) is a ripple current component
varying by the inductances L1, L2 of the electromagnets 2, 3
and containing displacement information on the rotor 1. Here,
ir1-ir2 is generally expressed in formula (4).

ir1-ir2=k{(1/L1)-(1/L2)} (4)

[0136]The k in formula (4) denotes a constant determined by the carrier
frequency fc and the PWM drive voltage E.

[0137]Here, when the rotor 1 is in the center between the electromagnet 2
and the electromagnet 3, then the gap between the electromagnets 2, 3 and
the rotor 1 is set as X0, and the inductance as L0. The formula
(5) can then be established when the rotor 1 was displaced by a tiny
amount x toward the electromagnet 3.

[0139]As shown in FIG. 1, the magnetic bearing device of the first
embodiment, detects the excitation current i1 and excitation current
i2 and by allowing the sum of their respective signals to pass
through a band-pass filter 205 whose center frequency is the carrier
frequency fc, removes the control current ic of formula (3), and extracts
the ripple current component or in other words, the displacement
information component ir1-ir2. This displacement information
component is obtained as an AM modulation signal equal to the carrier
frequency fc, versus the displacement of the rotor 1, and is demodulated
by the demodulation means 206, to obtain the displacement signal v0.
The demodulation means 206 demodulates the displacement information
component of the AM modulation signal according to the pulse signal Sc of
a frequency equal to the PWM carrier frequency fc obtained from the
oscillator 210.

[0140]The displacement signal v0 obtained by the demodulation means
206 is fed back and the comparator 212 compares it with the levitating
target position signal r, and inputs a differential signal ve to the
digital processor means 209, to obtain the desired control signal u' for
supporting the rotor 1 in a stable, non-contact levitating state. This
control signal u' is then input to the driver 204 to start the excitation
i1, i2 current flow and excite the electromagnets, to obtain
the magnetic force required for supporting the rotor 1 in a non-contact
levitating state. The displacement signal v0 is adjusted so that
v0=Ksx for a displacement x. Here, Ks denotes the specified
constant.

[0141]Now, when the rotor 1 is in the center between the electromagnet 2
and the electromagnet 3, the gap between the electromagnets 2, 3 and the
rotor 1 is set as X0 and the inductance is set as L0; and when
the rotor 1 is displaced by a tiny amount x towards the electromagnet 3,
then the magnetic force f1 and the magnetic force f2 that the
electromagnets 2, 3 respectively exert on the rotor 1 can generally be
expressed as the formula (6).

f1=k0{i1/(X0+x)}2

f2=k0{i2/(X0-x)}2 (6)

[0142]The magnetic force f1 and f2 as shown in formula (6) are
non-linear versus the excitation currents i1, i2 and
displacement x. As well known, a non-linear control system complicatedly
behaves and a linear control system, a simpler system, is preferable.
Here, the k0 in formula (6) denotes a constant determined by the
shape of the electromagnet core and the number of coil windings.

[0143]Therefore, as shown in FIG. 1, in the first embodiment the deviation
signal ve is subtracted from the levitating target position signal r in
the digital processor means 209, and the compensation signal u along with
estimated displacement signal xest obtained resulting from
multiplying the subtracted signal by 1/ks is input to the linearization
means 208, and linearization is then performed. The linearization means
208 processes according to formula (7), and outputs the control signal
u'.

u'=sign(u)Km(X0+|xest|)(u)1/2 (7)

[0144]Here, Km denotes the specified constant, and the sign (u) denote the
sign of the compensation signal u. The relationship between the
displacement x and the control signal u' is linearized through the above
processing. The output from the processing in the digital processing
means 209 is obtained via digital processing in DSP.

[0145]The inductances L1, L2 of the electromagnets 2, 3
generally change not only with the rotor 1 displacement but also with the
control current ic flowing in the electro magnets. The displacement
signal v0 obtained from the demodulation means 206 therefore
frequently contains a displacement error. To eliminate this displacement
error, a method can be used that detects the excitation currents i1,
i2, predicts the displacement error based on the control current
component signal obtained by removing the ripple current components by
allowing the mutually subtracted signals to pass through a low-pass
filter etc., and removes the predicted error from the displacement signal
v0.

[0146]In FIG. 13 and FIG. 14, an embodiment with a specific configuration
is shown. In FIG. 13, a difference between the current i1 and
i2 is detected by the subtractor 243 and the ripple current is
removed by a low pass filter (not shown) to detect the control current
ic. This signal is allowed to pass the cascade circuit of the filter
242 the amplifier 241. Here, the transfer characteristics of the filter
242 is set so that it agrees to the overall transfer characteristics Gz
obtained by putting the adder 211, BPF 205 and demodulation means 206
together. The amplifier 241 is set at a specific gain determined based on
the PWM power source voltage, PWM carrier frequency and a varying degree
of the inductances L1, L2 caused by the excitation current etc.
Thus, the predicted displacement error signal is output from the
amplifier 241. The output signal is subtracted from the output signal of
the demodulation means 206 and the displacement error signal is removed.
Here, the amplifier 241 does not have to be prepared separately from the
filter 242 but an amplifier and a filter may be integrated into a
filter-amplifier. Further, if a necessary level displacement error signal
can be obtained only with a filter, an amplifier is not required.

[0147]In FIG. 14, the subtracting process is carried out after the
comparator 212, and the same effect is obtained as in FIG. 13.

[0148]Another method is to employ an AM modulation to modulate the
displacement error component of the signal predicted from the control
signal u', and then detect the excitation currents i1, i2, and
add the currents i1 and i2 to obtain the summed AM modulated
signal from which the AM modulated signal of the displacement error
component may be subtracted. In FIG. 15 and FIG. 16, an embodiment with a
specific configuration is shown. In FIG. 15, a filter 246 with the
characteristics equivalent to the transfer characteristics Gd of the
driver 204, an amplifier 247 to amplify with a specific gain determined
based on the PWM power source voltage, PWM carrier frequency and a
varying degree of the inductances L1, L2 caused by the
excitation current etc., an AM modulator 248 to AM modulate with carrier
frequency fc are arranged between the input terminal of the driver 204
and the adder-subtractor 249 to remove the displacement error signal.

[0149]In FIG. 16, a filter 246 with the characteristics equivalent to the
transfer characteristics Gd obtained by integrating the transfer
characteristics of the driver 204 and those of the linearization means
208, an amplifier 247 to amplify with a specific gain determined based on
the PWM power source voltage, PWM carrier frequency and a varying degree
of the inductances L1, L2 caused by the excitation current
etc., an AM modulator 248 to AM modulate with carrier frequency fc are
arranged between the output terminal of the compensation means 207 and
the adder-subtractor 249 to remove the displacement error signal.

[0150]Here, the same as FIGS. 13 and 14, the amplifier 247 does not have
to be prepared separately from the filter 246 but an amplifier and a
filter may be integrated into a filter-amplifier. Further, there is
possibly an embodiment where an amplifier is not required.

[0151]In the first embodiment of the magnetic bearing device, the
unnecessary energy losses due to copper loss in the electromagnet coil
can in this way be minimized by allowing electrical current flow in just
one of the opposing electromagnets 2, 3.

[0152]Examples were utilized to describe the first embodiment of the
present invention; however, the present invention is not limited to that
embodiment. Various changes and adaptations are possible within the scope
of the patent claims as well as within the scope of the technical
concepts in the specifications and drawings. For example, the AC current
transfer means in the above embodiment for eliminating DC from excitation
current utilized a serially connected coil Ls and a capacitor Cs as the
resonator means 225 set to resonate at the same frequency as the carrier
frequency. However, the resonator means need not utilize such a passive
filter and may utilize an active filter instead. Moreover, an electronic
volume may be utilized for lowering the impedance in the vicinity of the
carrier frequency.

[0153]The second embodiment of the present invention is described next
with reference to FIG. 10 through FIG. 12 and FIG. 17. FIG. 10 is a block
diagram showing an example of the magnetic bearing device of this
embodiment of the invention. The reference numeral 1 denotes the rotor.
The rotor 1 or in other words a rotating piece made up of a magnetic
piece, is interposed between a pair of electromagnets 2, 3 on opposite
sides to support the rotor 1 in a non-contact, levitating state. These
electromagnets 2, 3 support the rotor 1 in one degree of freedom. Usually
multiple pairs of magnets are preferably utilized to support the rotor 1
in a levitating state in 5 degrees of freedom in directions other than
rotor axial rotation direction. Here, however for purposes of simplicity
only one degree of freedom is described the same as in the first
embodiment. The motor for rotating the rotor 1 is also omitted and only
the magnetic bearing is described. FIG. 11 is a drawing showing the
waveforms for signals from each section of this magnetic bearing device.
FIG. 12 is a drawing showing the relation between the excitation current
and the inductance of the electromagnets 2, 3. FIG. 17 shows an example
of the structure of the driver 108 in FIG. 10. In this second embodiment,
unlike the first embodiment, resistance values such as for the
electromagnet and cable are considered.

[0154]In FIG. 17, the driver 108 is a PWM type driver for supplying an
excitation current to the electromagnets 2, 3. This driver 108 supplies
the excitation currents i1, i2 to the electromagnets 2, 3 by
applying PWM voltages PWM1, PWM2 made up of pulse signals at
frequency fc generated by the oscillator 112 as the carrier frequency.

[0155]The driver 108 includes a PWM power supply for generating PWM drive
voltages E not shown in the drawing, and a bias power supply 124 for
generating a bias voltage Vb. The bias voltage Vb is utilized to supply a
direct current (DC) bias current Ib to the electromagnets 2, 3. This bias
current Ib makes the relation between the excitations current i1,
i2 supplied to electromagnets 2, 3 and the magnetic force applied to
the rotor 1 linear. Here, setting the DC resistance value of the
electromagnets 2, 3 and the cable as R, allows expressing the bias
current Ib as shown in formula (8).

Ib=Vb/(2R) (8)

[0156]The driver 108, including PWM regulator 123, supplies a control
current i c to the electromagnets 2, 3 superposed by a bias current Ib in
the form (Ib+i c, Ib-c), by controlling the duty ratio of the PWM
voltages based on the control signal u from the compensation means 107.
The PWM voltages PWM1 and PWM2 applied to the electromagnets 2,
3 at this time are controlled to mutually reciprocate the duty ratios so
that if a control current Ib+ic flows in electromagnet 2, then a control
current Ib-ic flows in the electromagnet 3.

[0157]The ripple currents ir1, ir2 are superposed onto the
excitation current supplied to the electromagnets 2, 3 just as described
for the first embodiment. These ripple currents ir1, ir2 are
generated by the PWM voltages PWM1 and PWM2 and vary according
to the impedance of the electromagnets 2, 3. The fundamental frequency of
this PWM voltage is sufficiently high since it is a carrier frequency fc,
and if taking just the ripple current into account, then impedance of the
electromagnets 2, 3 can be regarded as just the inductance component.
Therefore, if the PWM voltage is a fixed voltage, then the amplitude of
the ripple currents ir1, ir2 is dependent only on the
inductance L1, L2 of the electromagnets 2, 3. The duty ratios
of the PWM voltages PWM1 and PWM2 are also controlled to
mutually reciprocate each other to generate an amplitude that makes the
ripple currents ir1, ir2 rise and fall in reciprocal
directions. The excitation currents i1, i2 supplied to the
electromagnets 2, 3 therefore are derived as shown in formula (9).

i1=Ib+ic+ir1

i2=Ib-ic-ir2 (9)

[0158]So that formula (10) can be derived from formula (9) as follows.

i1+i2=2Ib+(ir1-ir2) (10)

[0159]In formula (10), ir1-ir2 is the ripple current component,
and its frequency component is mainly the carrier frequency fc. This
ir1-ir2 therefore is the high frequency component. Removing
ir1-ir2 by using a low-pass filter allows obtaining formula
(11).

i1+i2=2Ib (11)

[0160]The information of bias current Ib can therefore be obtained from
formula (11).

[0161]When the resistance R of the electromagnets 2, 3 and the cables etc.
varies due to the temperature and cable length, then the bias current Ib
varies as can be seen from formula (8). The driver 108 then detects the
excitation currents i1, i2 using the formula (11), and after
summing their respective signals, feeds back to the PWM regulator 123 the
signal obtained by allowing it to pass through a low-pass filter, so that
the bias voltage power source 124 is controlled to maintain the bias
current Ib at a specified value.

[0162]FIG. 11 is a drawing showing the signal at each portion when the
control signal u varies to time as shown. The driver 108 outputs PWM
voltages PWM1, and PWM2 and applies them to the electromagnets
2 and 3 respectively. Voltages PWM1, and PWM2 are mutually of
reverse waveforms, assuming that the PWM driving voltage is E and the
bias voltage is Vb, they are E when the driver is ON and -E+Vb when the
driver is OFF. That is, The average voltages applied to the
electromagnets 2 and 3 are toward plus side depending on the bias voltage
and the control of the voltage Vb can let a desired bias current Ib
always flow in the electromagnets 2 and 3. Further, since voltages
PWM1, and PWM2 are mutually of reverse waveforms, if the driver
108 controls the duty ratio of the PWM signal based on the control signal
u to let the current Ib+ic flow in the electromagnet 2, the current Ib-ic
flows in the electromagnet 3. That is, the driver 108 controls the bias
current Ib with controlling the bias voltage Vb, and independently
controls the control current ic with controlling the duty ratio of the
PWM voltage respectively. Further, the ripple currents ir1 and
ir2 including the displacement information are respectively
superposed on the currents flowing in the electromagnets 2 and 3
depending on the PWM voltage.

[0163]Formula (12) can be established from formula (9) as follows.

i1-i2=2ic+(ir1+ir2) (12)

[0164]In formula (12), ir1+ir2 is the ripple current component
and removing it by a low-pass filter or other method yields formula (13).

i1-i2=2ic (13)

[0165]The control current ic information can be obtained from formula
(13). The driver 108 detects the excitation currents i1, i2
using the formula (13) and after subtraction feeds back a signal obtained
by allowing it to pass through a low-pass filter 122 to the PWM regulator
to control the duty ratio of the PWM voltage so that the control current
ic corresponding to the control signal u from the compensation means 107
is supplied to the electromagnets 2, 3, to induce excitation.

[0166]In formula (10), 2Ib denotes the bias current component of the
direct current. Removing the direct current component yields the formula
(14).

i1+i2=ir1-ir2 (14)

[0167]In formula (14), the ir1-ir2 varies according to the
inductances L1, L2 of the electromagnets 2, 3. This
ir1-ir2 is the ripple current component containing displacement
information of the rotor 1. This ir1-ir2 is expressed in
formula (15).

ir1-ir2=k{(1/L1)-(1/L2)} (15)

[0168]The k in formula (15) denotes a constant determined by the carrier
frequency fc and the PWM drive voltage E.

[0169]Here, when the rotor 1 is in the center between the electromagnet 2
and the electromagnet 3, then the gap between the electromagnet 2 and
electromagnet 3 and the rotor 1 is set as X0, and the inductance as
L0. The approximate formula (16) can then be established when the
electromagnets 2, 3 excitation current is a fixed bias current Ib, and
rotor 1 was displaced by a tiny amount x towards the electromagnet 3.

(1/L1)-(1/L2)=2x/(L0X0) (16)

[0170]The excitation current for electromagnets 2, 3 is actually a control
current ic superposed onto the bias current Ib and it varies. The
inductances of the electromagnets 2, 3 vary according to the excitation
current as well as the displacement of the rotor 1. This inductance
variation occurs because the magnetic properties of the electromagnet
core vary due to the excitation current. In other words, it is because
the core inductance of the electromagnet varies. If the variation width
of the excitation current Ib±ic of electromagnets 2, 3 is not very
large, and the rotor 1 displacement amount x is of tiny quantity, then
the inductance characteristics in this range of conditions will have a
slope of "-a", due to the control current ic. This state is shown in FIG.
12.

[0171]Therefore even if the rotor 1 is fixed at a gap X0, the
inductances will be L01, L02 if the electromagnet 2, 3
excitation currents are Ib+ic, Ib-ic, and cause respectively different
values to occur. This inductance variation is what makes the ripple
current component cause displacement detection errors. By taking this
inductance variation due to control current ic into account, the formula
(16) can be rewritten as follows as formula (16').

(1/L1)-(1/L2)=(2x/(L0-X0))+(2a/L02)ic (16')

[0172]The formula (17) can also be obtained from formulas (14), (15),
(16')

i1+i2=(2k/(L0-X0))x+(2ak/L02)ic (17)

[0173]Here, "a" is found in advance by calculation or by actual
measurement.

[0174]In the present invention, the ripple detection means 104 including a
transformer 113 is utilized to obtain displacement information from the
excitation currents i1, i2 in the electromagnets 2, 3 subjected
to excitation, by using formula (17). The transformer 113 in this ripple
detection means 104 has windings T1, T2 so as to sum the
excitation currents i1, i2 by utilizing electromagnetic
induction. The inductance of windings T1, T2 is set to be
considerably smaller than the inductance of the electromagnets 2, 3. A
winding T4 on this transformer 113 is wound so as to increase the
sum of the excitation currents i1, i2 obtained from T1,
T2 by a specified scaling factor b and output it.

[0175]The direct current component is at this time removed by the winding
T4, and the displacement information signal contained in the ripple
current component is extracted. The band-pass filter effect is obtained
to extract the ripple current component only at the specified frequency
band by connecting a capacitor 114 and a resistance 115 in parallel
across both terminals of the winding T4. The specified frequency
band for extraction is set in the vicinity of the carrier frequency fc
which is the main frequency of the ripple current component. The ripple
detection means 104 acquires the displacement detection signal from the
excitation currents i1, i2, and outputs it as an AM modulation
signal of the carrier frequency fc.

[0176]The signal output from the ripple detection means 104 is input to
the band-pass filter (BPF) 105, surplus noise is removed, and the
displacement modulation signal Vx' is acquired. This displacement
modulation signal Vx' is expressed by formula (18).

Vx'=b(i1+i2) (18)

[0177]The formula (19) is obtained from the formula (17) and (18).

Vx'=Ksx+aic (19)

[0178]Here, Ks=2bk/(L0X0) and a=2abk/L02 are
constants. The excitation currents i1, i2 are detected
according to this formula (19). Here it can be seen that the displacement
differential component aic that varies due to displacement information
component Ksx and the control current ic, is contained in the
displacement modulation signal Vx' which is acquired via the bandpass
filter 105 and ripple detection means 104.

[0179]If Gdr is set as the transfer characteristics from the control
signal u to be output of the compensation means 107 and to be input to
the driver 108 to the control current ic, then a formula (20) can be
expressed from formula (19).

Vx'=Ksx+aGdru (20)

[0180]The Gdr transfer characteristic is generally not of a very high
order, and is the low-order low-pass filter characteristic. In the
present invention, in order to remove the displacement error signal aGdru
of formula (20), an estimated control current icest is acquired by
allowing the control signal u to pass through a filter means 109
containing a low pass filter with characteristics equivalent to the
transfer characteristic Gdr. This estimated control current icest is
increased a times by an amplifier 110 whose scale factor is equal to the
constant a, and the estimated displacement error signal xest is
obtained.

[0181]The AM modulator 111 amplitude-modulates the estimated displacement
error signal xest based on the carrier frequency fc, and inputs the
resulting signal into the winding T3 of transformer 113 of the
ripple detection means 104. The winding T3 of transformer 113 is
wound so as the current signal passing the winding T3 to be subtracted
from the summed signals of excitation currents i1, i2 acquired
from the windings T1, T2. The winding T4 can in this way
acquire a displacement information signal whose displacement error signal
component was accurately removed. The displacement modulation signal Vx'
acquired via the band-pass filter 105 can therefore be rewritten from
formula (20) to (20') as follows.

Vx'≈Ksx (20')

[0182]The displacement modulation signal Vx' is demodulated by the
demodulation means 106 which is synchronized with the carrier frequency
fc, and the displacement signal Vx amplified with a specified gain (scale
factor c) is obtained. The formula (20') in this way becomes the formula
(21).

Vx'≈Kscx (21)

[0183]The two sets of curves at the bottom half of FIG. 11 are the control
signal u, the input signal having a wave form of the error differential
signal to be input to T3, the signal which the error is removed from and
the demodulated wave form Vx.

[0184]The displacement detection means 117 including the ripple detection
means 104, the band-pass filter 105, and the demodulation means 106 is in
this way able to output the displacement signal Vx. The displacement
signal Vx is therefore output as a signal where the displacement x is
amplified with gain of Ksc as described in the formula (21). This output
signal is then fed back and compared with the target levitating position
signal r in the comparator 116, so that by then acquiring the signal u
compensated in the compensation means 107, the rotor 1 can be stably
supported at a specified position in a non-contact levitating state.

[0185]The second embodiment of the present invention was described,
however, the present invention is not limited to the above embodiments,
and various changes and adaptations are possible within the scope of the
patent claims as well as the within the scope of the technical concepts
in the specifications and drawings.